They can start getting narky about 10-12 weeks for sure (it was at 12 weeks that our GP3 started dominance bonking the older pig)
Do they have enough hidey spaces (one each + a spare) cut 2 doors in each one. Boxes full of hay are a good 'Boredom Breaker' .
Do they go out in the run, sometimes a change of scenery helps them.
Enough room between their bowls and water bottle (ideally two of each but that's not feasible in most cages)
If they do get narky, that's a par-for-the-course part of growing up. Keep an eye on the one who's been injured, bathe the area in warm lightly salted water.
You could get a plant sprayer and if you catch them about to start, give a little squoosh of water (avoid their faces/eyes/ears though)
We usually shout "Oi" at them.
There are some good websites that give you the rundown of boar behaviour from snipey to full on boar aggression.
(The first winter I got GP1 and GP2 , I side-by-sided them at night in winter but they went out to the Pighouse by day. They had enough room to chill out. The next winter, I stuffed two boxes with hay, put a cover on the cage and made sure they had plenty of food. They were much much better)
So, give them as much room as you can. Enough boar spaces. Enough to occupy them.
When GP3 grew up, he subtly overtook GP1 as Alpha Boar. There was a bit of rumbling and yes, he did try to bonk him (GP1 was 
).
They rarely sat together, but they were 'there' for each other.